Crank shaft - The basics

The crankshaft turns the reciprocating motion into a circular motion. This is its only theoretical purpose to propel the vehicle. But in real terms there is much more to the crankshaft. As the crank does not rotate at a constant rpm without fluctuations vibrations are caused, which the crank partly has to counteract (the engine as a whole has to be designed to have the highest possible resonance frequency in order to stay in one piece).

Even then this is not the only reason why the engine can disintegrate. The pistons, gudgeon pins, rings, conrods, bolts, the flywheel and the crank are to be considered as one unit that must be balanced because otherwise another type of vibration will destroy the engine.

Initially the balancing is not too difficult, which unfortunately is changing when an increased rev range is required. Nevertheless balancing is necessary in any case and this is what the manufacturer does to every engine produced. Although not done to the degree, as it has to be done to high performance engines this balancing is adequate when the engine is operated in the manufacturers specified rev range.

People speak sometimes about a ?Zero balance?, which is ?offered? by some companies. The zero balance does not exist in real terms. But it is possible to come close to it. To be able to do it properly the balancing equipment must be capable to do this at a very high rpm. In other words it means that the higher the rpm at balancing the closer we can get to the zero balance point and the safer we are at increasing rpm. This is why in a specification sheet usually is written that the relevant crank is balanced to xxxx rpm.

Whenever an engine is to be fully rebuilt a balance should be on the list. This is even more important if the rebuilt is to increase the performance of the engine. Even more so it is an absolute necessity when pistons conrods or any other parts that are moving with the crank or the crank itself have been changed or modified.

Often cranks are offered to be polished. The polished crank is slightly lighter than the standard crank when polished and due to the smooth surfaces less oil will cling to it, which in turn helps throttle response. Often some more lightening can be done within reason and finally the feather edging. This additionally helps to lower the drag when hitting any oil.

Most of todays produced cranks are usually very well designed. They can take much more power than the engines are designed for. Therefore it is in most of the cases unnecessary to go through the expense of a billet crank. If a billet crank was considered it should be thought off, whether re-stroking and over boring the engine would not make sense at the same time (please note that this is not possible in all cases).

In high performance engines it is advisable to increase the journal clearances by about a thou in order to increase the oil cushion accordingly. To increase the oil pressure much over the standard pressure is not necessary as it is more important to have quantity than excessive pressure. In such cases the oil pump needs to be increased or alternatively a dry sump arrangement could be considered that is set to deliver the specified amount of oil.